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While theoretically this should be harmless for thoroughly robust IEqualityComparer implementations, SplitOrderedList appears to be able to invoke IEqualityComparer.Equals on one (or two) uninitialized values during normal use.
If you look at the current source code on trunk:
There are two constructors for Node that do not actually initialize the Node.Data or Node.SubKey members. Despite this, the IEqualityComparer is invoked on Node.SubKey in various scenarios (insert, search, etc) without anything that seems like it would detect that a node lacks a valid SubKey.
One of my users has observed behavior caused by this in my compiler when running in a multi-threaded environment on Mono 3.0.6 (built from source). I did some troubleshooting with him and ended up adding a special check to the key-type I was using to detect that an uninitialized Key had been passed to the IEqualityComparer implementation (Key's only constructor asserts that it is given non-null arguments and my code never uses the argumentless constructor or default() ). Adding the check caused the check to be triggered by SplitOrderedList, as you can see in the attachment text file.
Sadly I have not been able to easily come up with a reproducible test case for this problem due to the fact that it seems to only appear in multithreaded scenarios (and in fact the user was not able to reproduce it on all of his machines) - my only accessible Linux machine is single-core.
I'm not sure what the exact fix would be here. You could argue that all IEqualityComparer implementations should handle either argument (or both arguments) being default(T), but this is definitely something that I've never seen any of the MS BCL types ever do, so it's at least a big divergence from expected behavior - kind of troublesome since it's in a concurrent container, but I know these things happen sometimes. :) A simple fix might be to have a 'valid' flag stored in the Node type that is only set if the Node actually has a Key and a Value, and then to ensure that the EqualityComparer is only invoked if both nodes are valid. (If the nodes' validity doesn't match, they can't be equal because an invalid node doesn't have a key or a value).
Created attachment 3519 [details]
Log file from user showing special null check being triggered
Not sure why the log didn't attach the first time.
This is the key type implementation and comparer that contain the special null check:
Resolved in master